Oakdale farmers soon will have to start paying for their water based on how much they use, not just how much land they irrigate.

That’s not currently the case, but state regulations will force the Oakdale Irrigation District to replace its flat-rate pricing with a volume-based structure.

The goal is to give farmers a financial incentive to save water. The OID is among the last irrigation districts in the region to make the switch.

OID board members on Tuesday discussed how California’s Water Conservation Act of 2009 will impact Oakdale farmers and the district’s budget.

The bottom line: OID will have to buy new equipment to start measuring how much water each farm gets, and farmers are likely to get stuck paying for those meters.

Farmers also were warned they’ll likely pay more for water in the future. But they hardly could pay much less than they do right now.

OID hasn’t raised its flat-rate water prices in decades. The district’s farmers pay $19.50 per acre a year for irrigation, no matter how much water they use.

That’s a bargain compared with virtually anywhere else in California.

Example: A 10-acre OID farm using 5 acre-feet of water per year is charged $195 to irrigate its fields.

Similar-sized farms using the same amount of water would pay $819 in the Modesto Irrigation District and $760 in the Turlock Irrigation District.

But the big pricing difference is that OID farmers don’t get charged extra for using additional water. MID and TID farmers, by contrast, must pay more for every drop, so they are financially penalized if they don’t conserve.

“It’s time for a water rate increase, but nobody’s wanted to talk about it,” OID Director Jack Alpers said during the meeting.

Raising water rates isn’t something OID has had to do for more than a generation. Oakdale farmers, in fact, paid about 63 percent more for water back in 1979 than they do now. OID was able to lower its water rates over the years, thanks to income generated by its hydroelectric power plants.

The current drought, however, will reduce the amount of hydropower OID will be able to produce and sell this year. So, less of that money will be available to subsidize the cost of irrigation water.

How much OID water rates might increase and how much the state-mandated meters might cost was not discussed Tuesday. Instead, the district’s staff was directed to come back with proposals to convert OID from flat-rate to volume-based water pricing and for new water rates.